1. Field of the Invention
The present invention relates generally to optical fibers, and specifically to an optical fiber that can be terminated with de minimis reflection.
2. Description of Related Art
Optical fibers are widely used for transmitting optical signals because a near loss-less transmission can be achieved under ideal circumstances. One such fiber is a fused silica (SiO.sub.2) fiber having a core region doped with an element chosen to alter the fiber's index of refraction in that region, germanium being especially well suited for this function. The index of refraction of the pure fiber is typically 1.45, whereas the core region doped with, for example, 1 percent germanium is 1.456. As a consequence, light traveling down the core region will reflect at the boundary between the doped core and the pure silica rather than being transmitted through the boundary, which leads to a communication of essentially the entire signal from one end of the fiber to the other.
The fiber's core diameter is optimally related to the designated light wavelength to be transmitted, where a 4 micron diameter equates roughly to an 800 nanometer wavelength of light and a 10 micron diameter corresponds to a 1550 nanometer wavelength of light. Depending upon the amount of germanium, the loss in a 1550 nanometer signal can range from 0.2 to 2 decibels per kilometer of fiber.
The fiber just described is a passive fiber in that it merely communicates an input signal from one end to the other. In addition to communicating optical signals, special fibers can also generate a light source by introducing an excitation signal into a specially doped fiber which in turn causes the fiber to emit a light characteristic of the dopant. These "active" fibers can be doped with one or more of the rare earth family of elements such as erbium or neodynium. For discussion purposes we will describe a fiber doped with erbium, although it is to be understood that the invention covers a fiber doped with any of the elements which have similar characteristics to those described herein. When erbium is pumped with a laser at the appropriate wavelengths, it emits a light in the 1530 to 1560 nanometer (nm) wavelength. Other dopants include neodymium which emits at 1.06 micron and praseodymium which emits in the 1.3 micron region. An erbium doped fiber will comprise approximately 50 to 500 parts per million (ppm), which is between 0.05% and 5% of the concentration of germanium in the passive fibers.
When an erbium doped fiber is supplied with a source of energy being pumped into the fiber, such as for example a 1480 nanometer (nm) laser diode, the electrons in the erbium absorb the energy and jump to a higher energy state. The stored energy is eventually released leading to a growing cascade of 1550 nm light traveling down the fiber. This light can be used to power an optical component such as a fiber gyro. The fiber gyro typically includes a detector to measure the electrical voltage generated by the incoming light. The electrical voltage is demodulated and the light output is measured, and the system is then adjusted based on the measurements. A product of the operation of the fiber gyro is that some of the 1550 nm incoming light is reflected from the fiber gyro back down the erbium fiber. It is estimated that approximately between 0.1% and 4% of the original light is reflected back into the erbium fiber under normal conditions.
This reflected light enters the erbium doped region, which acts as a high gain device. Light which enters the fiber travels through it until it reaches the far end of the fiber, where the light is partially transmitted and partially reflected back down the fiber into the erbium doped region. The feedback from both the fiber gyro and the reflection at the fiber can create an unstable condition for the high gain EDF. If the end of the fiber provides a low reflectance of the incident light, such as less than 10.sup.-6, then the system will generally be stable and performance will not degrade. However, if the reflectance of the incident light is greater than 10.sup.-4, oscillations will occur which will degrade the output to the point where the system becomes unproductive. The problem, then, is to attenuate the amount of reflected light to at least less than 10.sup.-5 and preferably to less than 10.sup.-6.
The index of refraction is approximately 1.45 for the fiber and 1.0 for air. A typical internal reflection for a fiber end cut perpendicular to its axis would yield a 4% reflection or 4.times.10.sup.-2. This reflectance is well outside normal operating parameters for fiber systems and quickly results in detrimental oscillations in the system.
To decrease the reflectance at the end of a fiber, the fiber 10 may be cleaved at the end 12 at approximately 15 to 20 degrees, as shown in FIG. 1, which can decrease the reflectance to appropriate limits under ideal conditions. Light (indicated by arrow 14) traveling in the core 20 reaches end 12, and some light 16 is transmitted through the fiber while other light 18 is reflected back into the fiber. However, the light 18 reflected back into the fiber is deflected to a steeper angle due to the inclination of the end 12. This causes the light to exit the core 20 because the light 18 can no longer reflect down the core 20. This occurs because propagation of the reflected light does not occur if the light incident on the core interface is greater than approximately 4 to 5 degrees. Reflectance on the order of 10.sup.-6 can be achieved by this method only under ideal conditions.
The problem with this solution is that it is difficult to achieve a smooth cut in a fiber that is only 80 microns to 125 microns in diameter. Aberrations in the cleaved surface can result in excessive light being reflected back down the fiber, which leads to feedback. Moreover, even if a smooth surface is created initially, installation of the fiber often leads to damage at the surface where hackles or chips can develop and accumulate. Furthermore, it is impossible to verify the integrity of the surface of the fiber's end until the fiber is installed in a system and the electrical output from the fiber gyro is demodulated. If the fiber turns out to be unsatisfactory, the entire system must be disassembled and the fiber refinished or replaced, at considerable expense to the user. The inability to verify the condition of the fiber's end until the system is assembled and operating is a major drawback of the prior art.
To protect the cleaved fibers, it is known to insert the end into a precision capillary glass tube having an inner diameter just slightly larger than the outer diameter of the fiber, which serves to protect the end of the fiber. Epoxy is added to fill the gap so that the fiber end is less susceptible to jarring or damage. This process is expensive, and fibers are frequently damaged in the process of insertion into the glass tubes. Once inserted into the glass tube, there is still no assurance that the fiber will perform properly until the system is assembled and the output is checked. Reassembling and disassembling the system also leads to damaged fibers, and the use and testing of the fibers becomes a prominent cost of the system.
What is needed in the art is an optical fiber which is not dependent upon the condition of the end surface or the quality of the cleave in order to achieve the desired reflectance conditions.